Add group epic boards API endpoint

Related to #385903 (closed)

What does this MR do and why?

Adds the following endpoints to retrieve epic boards for a given group:

GET /groups/:id/epic_boards

GET /groups/:id/epic_boards/:board_id

We will expose the board lists in following iterations, similarly to the issue boards API endpoints.

How to set up and validate locally

1. Create a private group with multiple labels and epics
2. Visit the group's epic boards in http://gdk.test:3000/groups/{GROUP_ID}/-/epic_boards and create more boards with lists
3. Test the following endpoints with a guest and a non-member:

export GUEST_PAT=<guest user api token>
export NON_MEMBER_PAT=<non-member api token>

# Fetch multiple boards

curl --header "PRIVATE-TOKEN: NON_MEMBER_PAT" "http://gdk.test:3000/api/v4/groups/{GROUP_ID}/epic_boards"
# returns {"message":"404 Group Not Found"}

curl --header "PRIVATE-TOKEN: GUEST_PAT" "http://gdk.test:3000/api/v4/groups/{GROUP_ID}/epic_boards"
# returns the correct response containing all epic boards for the group

curl --header "PRIVATE-TOKEN: GUEST_PAT" "http://gdk.test:3000/api/v4/groups/{GROUP_ID}/epic_boards?page=1&per_page=1"
# returns the correct paginated response

curl --header "PRIVATE-TOKEN: GUEST_PAT" "http://gdk.test:3000/api/v4/groups/{GROUP_PATH}/epic_boards"
# returns the correct response containing all epic boards when using the group path

# Fetch a single board by its ID

curl --header "PRIVATE-TOKEN: GUEST_PAT" "http://gdk.test:3000/api/v4/groups/{GROUP_ID}/epic_boards/{BOARD_ID}"
# returns the correct response containing the single board

curl --header "PRIVATE-TOKEN: NON_MEMBER_PAT" "http://gdk.test:3000/api/v4/groups/{GROUP_ID}/epic_boards/{BOARD_ID}"
# returns {"message":"404 Group Not Found"}

Example response for a single epic board

{
  "id": 8,
  "name": "Test 2",
  "hide_backlog_list": true,
  "hide_closed_list": true,
  "group": {
    "id": 204,
    "web_url": "http://127.0.0.1:3000/groups/group-with-boards",
    "name": "Group With Boards"
  },
  "labels": [
    {
      "id": 118,
      "name": "green",
      "description": "",
      "description_html": "",
      "text_color": "#FFFFFF",
      "color": "#00b140"
    }
  ],
  "lists": [
    {
      "id": 22,
      "label": null,
      "position": null
    },
    {
      "id": 24,
      "label": {
        "id": 119,
        "name": "pink",
        "description": "",
        "description_html": "",
        "text_color": "#FFFFFF",
        "color": "#c21e56"
      },
      "position": 0
    },
    {
      "id": 23,
      "label": null,
      "position": null
    }
  ]
}

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